The major currents of conventional methods for manufacturing high-purity polycrystalline silicon are the Siemens method and the monosilane method. These methods are methods in which a starting-material silane gas is supplied to the interior of a high-temperature reaction furnace from a nozzle installed in the bottom part of this sealed reaction furnace, and polycrystalline silicon is manufactured by the deposition or growth of silicon effected by the pyrolysis or hydrogen reduction of the starting-material gas on the surface of a solid silicon rod (silicon seed rod or core) disposed inside the furnace.
The starting-material silane gas that is used is a chlorosilane expressed by the formula ClnSiH4-n (n is an integer from 0 to 4) which is refined to a high purity; this may be monosilane, trichlorosilane or tetrachlorosilane used alone, or may be a mixture of these compounds. However, in the Siemens method, trichlorosilane (n=3) is the main compound used, while in the monosilane method, monosilane (n=0) is the main compound used. The silicon obtained by the pyrolysis or hydrogen reduction of these starting-material gases at a high temperature has the same composition as the silicon seed rod (hereafter referred to as the Si seed rod) set beforehand inside the furnace; accordingly, this silicon has a uniform high purity from the center part to the outer circumferential part. Since this silicon has a purity that is essential in the semiconductor industry, it is called the semiconductor grade polycrystalline silicon (SEG-Si).
In spite of the fact that the Siemens method belongs to a pre-war invention, this method makes it possible to obtain uniform high-purity polycrystalline silicon with constancy; accordingly, the basic conditions of this method remain unchanged today. The reaction equation in the case of trichlorosilane is shown below.SIHCl3→(thermal decomposition)→Si(polycrystalline silicon)
Quartz glass has been used as the material of the initial-stage furnace (bell jar) in the Siemens method. However, as the demand for polycrystalline silicon has grown, the size of the reaction furnace has been increased in order to increase the productivity, and currently, metallic bell jars made of corrosion-resistant metals such as carbon steels and high-nickel steels are at the startline of their use. Furthermore, modifications such as mirror finishing, silver plating or the like of the inside surfaces of the furnace have been used as a means for facilitating easy and uniform temperature control inside the furnace and preventing the loss of thermal energy due to heat radiation (Patent Document 1).
Meanwhile, with an increase in the size of the reaction furnace, the number of seed rods has increased, and the length of the rods has also increased. Accordingly, the yield of high-purity product with good quality and a uniform size has degraded. Furthermore, there have been various proposals in the past regarding improvements in the starting-material gas supply nozzle structure, improvements in the position and structure of the waste gas discharge port (Patent Document 2, Patent Document 3) and elsewhere as a measure for improving the uniformity and smoothness of the rod shape.
In the monosilane method, monosilane (SiH4) is the starting material. The thermal decomposition reaction of monosilane differs from the thermal decomposition in the Siemens method (thermal decomposition temperature 759 to 950° C.) in that this reaction is accompanied by the production of a silicon dust. As in the Siemens method, the core material is an Si rod. Since the polycrystalline silicon obtained uses monosilane as the starting material, there is no chlorine contamination, so that the purity is higher than in the Siemens method; this polycrystalline silicon is used mainly as a base material in the manufacture of single crystals by the FZ (floating zone melting) method.
Meanwhile, with regard to the methods for the manufacture of polycrystalline silicon for use in solar cells, various methods for purifying metallic silicon, fluidized bed reactions and the like have been proposed; however, for the reasons described later, these methods have not been adapted for practical use.
In the Siemens method, purified silane gas is subjected to the pyrolysis or hydrogen reduction inside a sealed vessel that is resistant to contamination, so that SEG-Si is formed. Accordingly, this method offers the advantage of easily producing a high-purity product. However, since the diameter of the Si seed rod on which the silicon polycrystals are deposited following the pyrolysis is extremely slender at around 5 mm, the surface area which is available for deposition in the initial stage of the reaction is so small that this method suffers from the drawback of a slow deposition rate.
Furthermore, the resistivity of the Si (Si rod) is large, i.e., 1 kiloohm-cm or higher so that it is difficult to pass current through the rod at room temperature. Accordingly, when the reaction is initiated, it is necessary to heat the seed rod from outside by means of a preparatory heating device up to a temperature that enables heating by passage of electrical current. Not only is a special high-voltage power supply device required for this heating, but a large amount of electric energy is consumed; as a result, this is a factor that increases the cost.
A method using a slender core rod comprising a metal such as Mo, W, Ta, Nb or the like with a high recrystallization temperature instead of the Si seed rod used in the Siemens method is also widely known (Patent Document 4). However, in cases where (for example) a simple metal of Mo is used, because of thermal expansion, electrical vibrations or the like caused by electric heating, or if the reaction temperature exceeds the recrystallization temperature (900° C.) of Mo the rod becomes flexible, and then forms recrystallized particles of the cubic system, becoming embrittled so that deformation readily occurs in response to the load, thus causing the silicon product obtained to assume a bent shape, sometimes resulting in contact with adjacent carrier core rods, so that the electric circuit is short-circuited, causing the reaction to be interrupted.
In the case of a Ta simple metal, embrittlement occurs as a result of a reaction with hydrogen at a high temperature, and in the case of Nb and W simple metals, the recrystallization temperature is 1150° C. or higher, but the impact strength drops, so that both metals suffer from problems relative to the strength.
Furthermore, in the case of SEG-Si obtained by using a slender core rod comprising metals such as Mo, W, Ta, Nb or the like, the core part must be removed by some method following completion of the reaction. Accordingly, it is recommended to use a “slender” core rod. However, since the core part that is embrittled following completion of the reaction is brittle and tends to crumble, it is difficult to remove this core part completely from the SEG-Si under visual inspection. Moreover, the following problem has also been encountered. Namely, these metals cause diffusion into the silicon that is deposited and grown, thus causing a deterioration in the purity of the product. Furthermore, a product that has once been used cannot be reutilized, so that the use of these expensive carriers is likewise not a satisfactory method in respect of cost, and has not reached the stage of practical use.
On the other hand, except for the high cost, the Siemens method is an indispensable technology for the manufacture of high-purity polycrystalline silicon. Accordingly, it will be understood that, if the productivity of the initial stage of the reaction in the Siemens method can be improved as described above, high-purity polycrystalline silicon can easily be obtained at low costs.
Since the polycrystalline silicon obtained by the monosilane method uses monosilane as the starting material, there is no chlorine contamination, and this polycrystalline silicon has a higher purity than that obtained by the Siemens method, and is used mainly as the base material for manufacturing single crystals by the FZ method. Products manufactured by the FZ method have a uniform diameter, and have a large diameter that is as close as possible to the diameter available when a single crystal is formed. Furthermore, such products contain no impurities such as infusible powders or the like, and in cases where a product with no bending is required, various modified techniques have been proposed for this purpose. For example, a method in which the gas flow rate through the furnace is stipulated in order to remove the boundary film that accumulates around the heating filament in order to accelerate the deposition of silicon (Patent Document 5), a method in which the reaction gas is fed over the cooling wall of a dust catcher together with the associated silicon powder in order to prevent deposition and admixture of an infusible powder (Patent Document 6), a method in which a major portion of the reaction mixture leaving the decomposition vessel is recycled into the supply line of the decomposition vessel in order to accomplish the decomposition of monosilane at an effective rate (Patent Document 7), a method in which the filament core wire interconnection bridge is constructed from tantalum, molybdenum, tungsten or zirconium so that the temperature does not become excessively high during the passage of electric current through the system (Patent Document 8) and others have been proposed.
However, in cases where the member is constructed from one of the metals cited in Patent Document 8, as is described above, recrystallization occurs at 1200° C. or higher, so that the member becomes brittle, thus making reutilization impossible.
Furthermore, monosilane readily takes fire; accordingly, not only is an extensive security apparatuses are required for the handling of this gas, but, since a fine powder is produced as a by-product during the reaction, the yield is low, and the drawback of a high manufacturing cost is encountered.
Various methods were proposed and tested in the past as the method for the exclusive manufacture of a (high-purity) polycrystalline silicon as the base material for use in solar cells. The ultimate object of such methods is to provide a high-quality product at low costs. In particular, although there is a desire for an inexpensive high-purity silicon base material exclusively for use in solar cells, there is regrettably no such exclusively usable base material source to be found.
Currently, the polycrystalline silicon base material that is used in solar cells is found in “low-grade SEG-Si” obtained as a by-product from processes manufacturing semiconductor-grade polycrystalline silicon or IC-grade silicon wafers, lump-form scraps (tops, tail portions, crystal side surface shavings of single crystal boules, crucible residues) or Si wafer scraps. However, there are limits to the amount of by-product scraps available, and in recent years, this amount has tended to decrease, so that the guaranteed supply sources with constancy for polycrystalline silicon in the development of solar cells has become a major problem.
In order to achieve a low cost, it is necessary that the starting material be inexpensive, and there have been many attempts to ensure this. One of these attempts is to purify metallic silicon (MG-Si) or by-product silicon from the semiconductor industry. For example, methods in which purification is accomplished by spraying a plasma jet gas at the surface of molten silicon with a plasma gas (Patent Document 9, Patent Document 10, Patent Document 11), a method using a direct-current arc furnace (Patent Document 12), and a method utilizing an electron beam, are known. Furthermore, numerous means have been proposed such as a method in which silicon debris discarded from the semiconductor industry are purified by a unidirectional solidification treatment (Patent Document 13), methods in which purification is accomplished by adding an inert gas and an active gas or a powder such as CaO or the like to molten silicon (Patent Document 14, Patent Document 15), methods in which purification is accomplished utilizing a difference in the boiling points with MG-Si placed under reduced pressure (Patent Document 16, Patent Document 17) and the like. However, no method producing satisfactory results using the starting materials obtained by these methods has yet been rendered to practice.
As for the reasons that it is difficult to purify molten silicon, the fact that silicon atoms readily form stable compounds with other atoms is also one factor; however, the main reason is that the p-type impurity B (boron) cannot easily be removed from silicon. The solid-liquid distribution (segregation) coefficient of B with respect to Si is close to 1, i.e., 0.81. Accordingly, separation and purification cannot be accomplished by a solid-liquid separation method such as unidirectional solidification or the like. Complete treatment of the system as a whole is difficult even if the boiling points differ, the blowing in of a gas or the like is utilized.
The method of purification relative to B is that it is a method in which “metallic silicon” is chemically reacted with “hydrochloric acid” to form a silane gas followed by distillation or adsorption to separate and purify the chlorinated boron formed by the reaction of B+HCl. The purified silane gas containing no impurities is then reduced to produce high-purity SEG-Si. The method of the present invention is usually called the gas purification method; the Siemens method and monosilane method are also each a variation of this method. Both of these methods involve numerous manufacturing processes and consume large amounts of energy; accordingly, the methods suffer from the drawback of a high cost, and there are problems in using these methods for the starting materials in solar cells.
Thus, with regard to the removal of B, gasification or cutting by distillation is the most reliable method. The gasification of B also results in the gasification of other impurity elements found as a solid solution in the Si, with these elements being removed by the difference in the boiling point. Methods known as the gasification methods besides the above-mentioned Siemens method and monosilane method include a fluidized bed reaction. In an externally heated reaction furnace, the starting materials (trichlorosilane+hydrogen) are supplied from the lower part of the reaction furnace, fine particles of Si are caused to be fluidized through the furnace, and the product is deposited to grow on the fluidized particles, so that polycrystalline silicon is produced. Following the reaction, the gas is discharged from the upper part of the furnace (Patent Document 18, Patent Document 19). The purity is six-nines (99.9999%) or higher, and satisfies the requirements for the solar cell grade.
In this method, a high-purity product can be obtained at a low cost; however, since the reaction is performed using an externally heated reaction furnace, Si is deposited and grown even on the inside surfaces or the reaction tube, so that a continuous reaction cannot be performed, and an increase in the size of the reaction tube presents a bottleneck, and the method has unfortunately not yet been adapted to practical use.
It is seen from the above that, if it is desired to obtain polycrystalline silicon having a high purity, the gasification method is superior. Furthermore, the difference in the starting materials used for semiconductors and solar cells is a difference in purity; in the former case, a purity of eleven-nines (11N) or higher is reportedly sufficient, while, in the latter case, it is sufficient if the purity is of six-nines (99.9999%) (5 orders of magnitude lower) or higher. Accordingly, it is seen that, if a method can be developed which satisfies the target purity, provides a cost that is levels lower than that of the former method, and enables constant supply, this can provide an “exclusive starting material source for solar cells”.    [Patent Document 1] Japanese Patent Application Publication No. 6-41369    [Patent Document 2] Japanese Patent Application Laid-Open No. 5-139891    [Patent Document 3] Japanese Patent Application Laid-Open No. 6-172093    [Patent Document 4] Japanese Patent Application Laid-Open No. 47-22827    [Patent Document 5] Japanese Patent Application Laid-Open No. 63-123806    [Patent Document 6] Japanese Patent Application Laid-Open No. 8-169797    [Patent Document 7] Japanese Patent Application Laid-Open No. 61-127617    [Patent Document 8] Japanese Patent Application Laid-Open No. 3-150298    [Patent Document 9] Japanese Patent Application Laid-Open No. 63-218506    [Patent Document 10] Japanese Patent Application Laid-Open No. 4-338108    [Patent Document 11] Japanese Patent Application Laid-Open No. 5-139713    [Patent Document 12] Japanese Patent Application Laid-Open No. 4-37602    [Patent Document 13] Japanese Patent Application Laid-Open No. 5-270814    [Patent Document 14] Japanese Patent Application Laid-Open No. 4-16504    [Patent Document 15] Japanese Patent Application Laid-Open No. 5-330815    [Patent Document 16] Japanese Patent Application Laid-Open No. 64-56311    [Patent Document 17] Japanese Patent Application Laid-Open No. 11-116229    [Patent Document 18] Japanese Patent Application Laid-Open No. 57-145020    [Patent Document 19] Japanese Patent Application Laid-Open No. 57-145021